1. Field of Application
The present invention relates to an electrically insulated type of switching element drive circuit, i.e., whereby a switching element is driven by a control circuit that is insulated from a source of a control command signal which selectively designates setting of the switching element to the on (i.e., conducting) and off (i.e., non-conducting) states, with the control circuit operating in accordance with that control command signal.
2. Description of Prior Art
A DC-DC converter circuit or DC-AC inverter circuit generally is made up of an input circuit which produces a control signal and a drive circuit which supplies drive signals to the control electrodes of switching elements in accordance with the control signal, with the reference (ground) potential of the input circuit usually being different from that of the drive circuit. This is true for example of a drive circuit which drives a high-side switching element of a DC-AC inverter circuit, in which case the drive voltage applied to the control electrode of the high-side switching element must be produced with reference to an output voltage of the inverter circuit, i.e., which appears at the junction between the high-side switching element and a low-side switching element. Moreover in many cases, the reference potential of the drive circuit for a low-side switching element of such a DC-AC inverter circuit is also different from the reference potential of the input circuit.
In such a case, an electrically insulated type of switching element drive circuit is used, i.e., in which the circuit which actually drives the control electrodes of the switching elements is electrically insulated from the input circuit which supplies the control signal, so that the drive circuit and the control signal supplying circuit can have respectively different signal reference potentials. Various types of electrically insulated type of switching element drive circuit are known, such as an AC transformer electrically insulated type of switching element drive circuit or a pulse transformer electrically insulated type of switching element drive circuit.
For example as described in Japanese patent No. 7-231250, the output voltage from the secondary coil of a pulse transformer is directly applied to the gate electrode of a MOS transistor used as a switching element. As an example of an AC transformer electrically insulated type of switching element drive circuit, the AC voltage from the secondary coil of a transformer is rectified to supply power to a control electrode drive circuit of a switching element. A control signal is supplied to that control electrode drive circuit via a photo-coupler system, for example, to be amplified therein, and a resultant signal being applied to the control electrode of the switching element.
With such a prior art type of electrically insulated type of switching element drive circuit, using an AC transformer or pulse transformer, power can be transferred from a control circuit (i.e., input circuit) to be used in controlling a switching element. However it is necessary for such a circuit to use transistors which have high power-handling capability and the overall size of the circuit becomes excessive due to the size and weight of the transformer, so that the overall scale of such a circuit is large